Plastic Antibodies - Designing Receptors for the Next Generation of Biosensors - page 104

3.5 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
3.6 The Future: Biologically Active MIP Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

1 Introduction: Antibodies—All That Glisters Is Not Gold

Molecular recognition plays an important role in diagnostics, catalysis, separation,
and drug development. Enzymes, antibodies, nucleic acids, and receptors are
widely applied in the fundamental study of molecular recognition phenomena, as
well as in the development of practical therapeutic or diagnostic systems [ 1 ].
Antibodies largely dominate in most of the commercial applications. To provide
an example, the global monoclonal antibodies (mAbs) market in 2009 was
evaluated as $40 Billion, with $30 Billion related to therapeutic applications
[ 2 , 3 ]. The global in vitro diagnostics (IVD) market, another application area for
antibodies and enzymes, has been estimated in 2010 as $44 Billion and is expected
to reach $52 Billion by the end of 2013. Among the key constituents of the IVD
market, the point-of-care (POC) segment holds the major part, followed by immu-
nochemistry and molecular diagnostics [ 2 , 4 ].
Antibodies are proteins which the immune system synthesizes to detect and
neutralize “non-self” substances (e.g., bacteria, viruses, and toxins), also known as
antigens [ 5 ]. The most commonly used immunoglobulins G (IgGs) possess a
Y-shape resulting from the arrangement of two longer (“heavy”) chains and two
shorter (“light”) chains, all stabilized by disulfide bonds, with an average molecular
weight of 150-160 kDa. The lower part of the “Y” is referred to as the Fc region,
and its role is to confer stability and drive the interactions with other components of
the immune system (e.g., effector mechanisms). The upper part of the “Y” is known
as the Fab region and contains the variable domains at which the antigen recogni-
tion and binding take place [ 6 ]. The antibody-antigen interaction is driven by a
precise combination of electrostatic, hydrogen bonding, van der Waals, and/or
hydrophobic forces, which results in extremely strong affinity [ 5 ]. Antibodies are
undoubtedly highly specific and selective for several kinds of chemical and
biological moieties and can be produced on a large scale [ 7 ]. Their industrial
production relies on the cultivation of modified mammalian cell lines [e.g., those
from Chinese-hamster ovary (CHO) and human embryonic kidney (HEK)-2930].
The manufacturing process is logistically difficult and expensive [ 6 , 8 , 9 ]. Further-
more, antibody production against small molecules requires chemical coupling
between haptens and a carrier protein in order to generate an immune response in
animals [ 7 ] and their purification involves several steps (especially for applications
in therapy), which affects the manufacturing costs [ 10 , 11 ]. Moreover, it is difficult
to generate antibodies against molecules such as immunosuppressant drugs or
toxins, because these chemicals act directly on the immune system and prevent
its natural response [ 12 , 13 ]. Additionally, being proteins, the characteristic problems
related to their usage are low stability and poor performance in organic solvents,